The brake system serves to slow down the motor vehicle, i.e., to provide a braking force acting on wheels of the motor vehicle. The braking force is applied to the wheels with the aid of the wheel brakes, i.e., the first wheel brake and the second wheel brake. In this case, the first wheel brake is provided for applying the braking force to a first of the wheels and the second wheel brake is provided for applying the braking force to a second of the wheels. If the motor vehicle has more than two wheels, then the brake system preferably has an additional wheel brake for at least one of the other wheels or all of the other wheels, by means of which the braking force may also be applied to the at least one additional wheel. For example, an actual brake pressure is applied to the wheel brakes upon actuating an operating element. The brake system is accordingly present as a service brake of the motor vehicle or forms at least a component of the service brake.
The brake system, for example, has a main brake cylinder, in which a main brake piston is movably arranged. Together with the main brake cylinder, the main brake piston bounds a brake fluid volume, which is variable, its size being dependent on the position of the main brake piston. The main brake piston is coupled to an operating element, which is present as a brake pedal, for example. By way of the operating element, a driver of the motor vehicle can adjust a desired braking force, which is called hereafter the specified braking force and which preferably stands in a fixed relation with a specified brake pressure.
The brake system is preferably present as an electrohydraulic brake system. This means that, in at least one operating mode of the brake system, upon actuating the operating element, the brake fluid present in the brake fluid volume does not directly provide the actual brake pressure—or at least a portion of this—applied to the first wheel brake and/or second wheel brake. Instead, it is provided that, upon actuating the operating element, a target brake pressure is to be determined, which can be provided with the aid of at least one sensor, which is coordinated with the operating element and/or the main brake piston and/or the main brake cylinder and/or a simulator cylinder, in which a simulator piston is movably arranged.
The sensor may be designed, for example, as a distance sensor or as a pressure sensor. In the first case, the sensor is used to determine the actuation path of the operating element by which the operating element has moved when actuated. In addition or alternatively, of course, the pressure present in the main brake cylinder can be determined by means of the sensor. From the quantities measured with the aid of the sensor, i.e., the distance and/or the pressure, for example, the target brake pressure is then determined. After this, an actual brake pressure corresponding to the target brake pressure is adjusted or applied to the first wheel brake and/or the second wheel brake.
The actual brake pressure is thereby provided by the brake pressure source, which is present, for example, in the form of a pump, especially an electrically operated pump. In the above-described operating mode of the brake system, accordingly, the brake fluid volume is not connected or at least not directly connected or fluidically connected, to the first wheel brake and/or the second wheel brake. Still, in order to provide the driver of the motor vehicle with a haptic feedback upon actuation of the operating element, the main brake cylinder is preferably coordinated with an—optional—braking force simulator. The latter has a simulator piston, which is movably arranged in a simulator cylinder and braced by a spring element against a wall of the simulator cylinder, and being thus spring-loaded.
The simulator piston, together with the simulator cylinder, bounds a simulator fluid volume, which is variable, analogously to the brake fluid volume, the size of the simulator fluid volume being dependent on the position of the simulator piston. The simulator fluid volume is fluidically connected to the brake fluid volume. Upon actuating the operating element, the brake fluid volume is decreased and brake fluid present in the brake fluid volume is supplied to the simulator fluid volume. Accordingly, the simulator fluid volume increases, which deflects the simulator piston against the spring force.
Depending on the spring force, which may depend on the deflection of the simulator piston, in the described operating mode, an opposing force acts on the operating element because of the fluidic connection between the simulator fluid volume and the brake fluid volume, and this is directed opposite to the operating force applied by the driver of the motor vehicle to the operating element. Accordingly, the driver receives by way of the operating element a haptic feedback, which is basically dependent on the deflection of the operating element from its starting position or position of rest.
In order to provide a fallback in event of a fault in the brake system, such as, for example, a loss of the brake pressure source, there is preferably a direct fluidic connection between the main brake cylinder and the first wheel brake and/or the second wheel brake. In this way, even given a defect in the brake system, an actual brake pressure can be built up on the first wheel brake and/or the second wheel brake upon actuating the operating element. But for this purpose, the driver must apply a substantially greater operating force to the operating element than is usual.